Broadband wireless access (BWA) systems are currently in development that will provide high speed wireless network services over relatively large regions. One such technology is embodied in the IEEE 802.16 wireless networking standard (IEEE Std 802.16-2004). Typically, such systems will utilize a number of distributed base stations to provide access services for users over an extended area. Each base station will provide services for subscriber stations within a corresponding coverage area or cell. Time division duplexing (TDD) is currently a preferred technique for managing communication between a base station and a subscriber station within a cell of a BWA system. That is, the base station and the subscriber station will transmit signals to one another using the same frequency, but the base station will transmit at a different time than the subscriber station. TDD has certain advantages over frequency division duplexing (FDD), where the base station transmits at a different frequency from the subscriber station.
One communication technique that has become a popular choice for use in broadband systems is orthogonal frequency division multiplexing (OFDM)/orthogonal frequency division multiple access (OFDMA). OFDM uses a plurality of relatively narrowband subcarriers to transmit data through a channel. Each subcarrier may be separately modulated with a corresponding data symbol within the transmitter. An “OFDM symbol” is then transmitted that includes all of the subcarriers and their corresponding symbols. OFDMA is a technique that may be used to provide multiple access in an OFDM system. In OFDMA, different subgroups of subcarriers within an OFDM symbol may be allocated to different users in the system. It has been shown that maximum spectral efficiency may be achieved in an OFDMA system when each of the cells in the system utilize the same frequency (i.e., frequency reuse 1). When TDD is being used in such a scenario, interference between base stations may occur if one base station is transmitting while another base station is receiving. Thus, if a portion of an uplink subframe in one cell occurs at the same time as a portion of a downlink subframe in an adjacent cell (and vice versa), then communication within both cells may be compromised. To prevent this from occurring, the base stations within a network may be synchronized so that uplink and downlink operations do not overlap.
It has been proposed that GPS receiver technology be provided within each of the base stations within a BWA system for use in providing synchronization between the base stations (see, e.g., IEEE Std 802.16-2004). While GPS receivers are capable of providing accurate synchronization, there are situations where their use may be limited. If it is desired to locate a base station within a building, for example, the GPS receiver may not be able to acquire an adequate number of satellite signals to generate accurate time information for use in synchronization. In addition, the inclusion of a GPS receiver in a base station may add significant cost to the base station. In some BWA system implementations, for example, the cell size may be relatively small (e.g., 0.3-1 km), thus requiring a large number of pico-base stations to cover a given area. If base stations are expensive, the cost of implementing such a system may be prohibitive. If base stations are available that do not include GPS receivers, then system implementation costs may be reduced significantly. For each of the above reasons, it may be desirable to develop alternative techniques for synchronizing the base stations of a BWA system.